One possible answer is the lack of start-ups with a commitment to creativity.
The last wave in the UK included shops like Leagas Delaney (that's Tim Delaney in the pic), Rainey Kelly, Howell Henry, Simons Palmer and - more recently - Mother. All great agencies.
The new wave consists of places like CHI, VCCP and DLKW. Very successful business-wise... but their ads smell like three-year old gorgonzola wrapped in a sock.
Plus, no creative start-ups means less pressure on the big agencies to be creative.
Any other theories?